Hydrogen-dominated atmospheres in low-mass planets.

Origin and Loss of nebula-captured hydrogen envelopes from "sub"- to "super-Earths" in the habitable zone of Sun-like stars
http://arxiv.org/abs/1401.2765

It seems that under some conditions, low-mass planets are unable to shed their hydrogen-dominated proto-atmospheres. This may explain some of the extremely low-density super-Earths we've been seeing from Kepler.

I quite like their discussion of the claims of habitability of the planets in the Kepler-62 system:

Quote :

From our results shown in Table 5, it is not comprehensible why these authors concluded that Kepler-62e and Kepler-62f should have lost their primordial or outgassed hydrogen envelopes despite the lack of an accurately measured mass for both ‘super-Earths’. If one assumes that both ‘super-Earths’ with their measured radii of ~1.5R_⊕ have an Earth-like rocky composition, one obtain masses of ~3–4M_⊕ (e.g., Sotin et al. 007). As one can see from Table 5, depending on the nebula properties and accretion rates, ‘super-Earths’ inside this mass range can capture hydrogen envelopes containing between hundreds and several thousands EO_H. Although, Kepler-62 is a K-type star which could have remained longer in the XUV-saturation than ~100 Myr, from our model results it is obvious that, contrary to the assumptions by Borucki et al. (2013), the XUV-powered hydrodynamic escape rates are most likely not sufficient for ‘super-Earths’ within this mass range to get rid of their hydrogen envelopes if they orbit inside the HZ of their host star.